1. Field of the Invention
The present invention relates to the art of sputtering, and more particularly to a composite sputtering cathode assembly having a plurality of targets and a sputtering apparatus having such a composite sputtering cathode assembly. More specifically, the present invention is concerned with the sputtering technology which is capable of forming a thin film in minute holes (hereafter the words "minute holes" include narrow slits) of a high aspect ratio with a good coverage using a composite sputtering cathode assembly.
2. Description of the Prior Art
Sputtering apparatus are widely used in the fabrication of electronic devices such as semiconductor devices and liquid crystal display units.
As shown in FIG. 10 of the accompanying drawings, a conventional sputtering apparatus 108 has a target 105 mounted on a cathode electrode 104 disposed in a vacuum housing 110, and a substrate 112 placed on a substrate holder 111 disposed in the vacuum housing 110 in confronting relation to the target 105. A plasma is generated on the surface of the target 105 to expel the material of the target 105 as sputtering particles which are deposited as a thin film on the surface of the substrate 112.
The substrate 112 and the target 105 are closely spaced from each other by a distance of about 6 cm. Therefore, the sputtering particles are applied to the surface of the substrate 112 at various angles as indicated by the arrow 120. Therefore, the minimum angle .theta. at which the sputtering particles are applied to the surface of the substrate 112 is very small at ends of the substrate 112.
Recent years have seen an increase in the degree of integration of electronic devices such as semiconductor devices. Because of such an increased degree of integration, minute holes defined in substrate surfaces of such electronic devices have increased aspect ratios (depth-to-diameter ratios).
When a thin film is deposited on a substrate having minute holes with a high aspect ratio by the conventional sputtering apparatus, a thin-film overhang is developed at the open ends of the minute holes, and no sufficient thin film is produced at the bottom ends of the minute holes.
One solution which has heretofore been proposed is shown in FIGS. 11 and 11a of the accompanying drawings. As shown in FIG. 11, the distance "d" between a target 205 and a substrate 212 is increased, and a high vacuum is developed to increase the mean free path of sputtering particles, for thereby increasing the minimum angle .theta. at which the sputtering particles are applied to the surface of the substrate 212.
However, the increased distance "d" between the target 205 and the substrate 212 is not sufficient to deposit a thin film on the bottom of a minute hole 215 at an end of the substrate 212 with a uniform coverage.
Specifically, sputtering particles expelled from a large surface area of the target 205, ranging from the right end to the left end as shown in FIG. 11, reach the substrate 212 as indicated by the arrows 221. As shown in FIG. 11a, on the right end of the substrate 212, more sputtering particles arrive at the right-hand side of the minute hole 215 as indicated by the arrows 221.sub.1, and less sputtering particles arrive at the left-hand side of the minute hole 215 as indicated by the arrow 221.sub.2. Therefore, a thin film 218 deposited on the bottom of the minute hole 215 is thicker at the right-hand side and thinner at the left-hand side, and hence the coverage by the thin film 218 of the bottom of the minute hole 215 is asymmetric (known as a step coverage).
The asymmetry of the coverage by the thin film 218 of the bottom of the minute hole 215 can be eliminated if the distance "d" between the target 205 and the substrate 212 is further increased to allow the sputtering particles to be applied perpendicularly to the substrate 212. However, the further increase in the distance "d" reduces the number of sputtering particles that reach the substrate 212, resulting in a reduction in the film growth rate.
FIG. 12 of the accompanying drawings shows another conventional sputtering apparatus 302. The conventional sputtering apparatus 302 has a target 305 mounted on a cathode electrode 304 disposed in a vacuum housing 310, and a collimator 307 disposed between the target 305 and a substrate holder 311 disposed in the vacuum housing 310 in confronting relation to the target 305. Sputtering particles expelled in various directions indicated by the arrow 320 from the target 305 are applied to the collimator 307. The collimator 307 passes only those sputtering particles that are directed perpendicularly as indicated by the arrow 327 to a substrate 312 supported on the substrate holder 311, and entraps those sputtering particles that are directed in other directions toward the substrate 312, for thereby depositing a thin film in minute holes in the substrate 312 with a uniform coverage at their bottom ends.
However, as the number of sputtering particles entrapped by the collimator 307 increases, the openings of the collimator 307 are constricted and progressively limit the number of sputtering particles that can travel through the collimator 307, resulting in a reduction in the film growth rate.
If the target 305 is increased in diameter, then the plasma developed on the surface of the target 305 suffers large density irregularities, and hence the sputtering rate on the surface of the target 305 differs greatly depending on the position on the surface of the target 305.